Can you damage your speakers by plugging bass port vent?

HI All,

I have ProAc K6 Signature driven by AR Ref 150SE with AR Ref 5SE preamp....Unfortunately I am now staying in an apartment building - all concrete slab walls, ceilings etc.... Bass is just horrible... This is just a temporary arrangement and so I don't want to spend any moeny on acoustic treatments etc...  Will be moving to a house in about 9 to 10 months.... However, in the meantime, I was thinking to plug the bottom firings woofer port vent with some foam or fabric. But worry that this may cause all the energy to bouce back inside and....after several months...cause some damage to either the woofer or the speaker cabinets.... 

Is this something to worry about or would you say plugging the vent won't cause any issues....

PS: I would probably experiment with various materials, like cloth or foam or whatever.... to get the right balance.

THANK YOU !!